A conventional revolution arrangement for the movable scroll in a conventional scroll-type compressor is, for example, disclosed in Japanese Laid Open Patent Publication No. 59-28082. As shown in FIG. 13, this prior art design has recess forming plates 33 and 34 inside a housing. The plate 33 is fixed by a plurality of pins 35 or the like on an end surface of a movable scroll 31. The plate 34 is fixed by the pins 35 on a surface of a fixed plate 32 that faces the scroll 31. Circular openings 36, 37 having the same diameters are provided on each recess forming plate 33, 34. The holes are arranged circumferentially about the plate at fixed intervals. Thus, a plurality of recesses 36 and 37 are defined by the openings in a circular pattern on the end surface of the movable scroll 31 and the facing surface of the fixed plate 32. A cylinderical coupling lug 38 is placed between each facing pair of recesses 36 and 37.
Both axial thrust loads and radial loads are applied to the movable scroll 31 when gas compressed. The thrust load is transmitted longitudinally through the coupling lug 38 between the opposing bottom surfaces of the recesses 36 and 37. The radial load is transmitted somewhat more radially through the coupling lug between the inner periperal surfaces of the recesses 36 and 37. With this arrangement, the movable scroll 31 is prevented from rotating about its axis while it is permitted to make an orbital motion relative to the a fixed scroll (not shown).
The fixed plate 32 has a central opening 39 which receives a needle bearing 41. An eccentric shaft 40 is coupled to the movable scroll 31 via the bearing 41. Since large loads are applied to the bearing 41 when the gas is compressed, it is preferably that the diameter thereof be as large as possible so as to improve its overall strength and abrasion resistance.
However, this prior art design has several drawbacks. The recesses 37 on the fixed plate 32 have the same large diameter as the recesses 36 on the movable scroll 31. Therefore, the solid portion of the fixed plate (that is the portions in which the recesses 37 are formed), must be relatively large in the radial direction. As a result, the central opening 39 must be relatively small in diameter which restricts the diameter of the bearing 41. Smaller diameter bearings have reduced mechanical strength which increases the likelihood of rattling and otherwise making noise.
Of course merely increasing the diameter of the central opening 39 is not an acceptable solution since such an increase would necessitate the overall enlargement of the compressor which is also undesirable.